Monitoring a biological body state of a driver during driving has attracted attention as a preventive measure against an accident or the like in recent years. The present applicant disclosed techniques of disposing a pressure sensor in a seat cushion section, obtaining and analyzing breech pulse waves, and determining a sleep prediction phenomenon in Patent Literatures 1 to 3.
Specifically, a maximum value and a minimum value of a time-series waveform of a breech pulse wave are obtained by a smoothing differentiation method of Savitzky and Golay, respectively. The maximum value and the minimum value are obtained for each 5 seconds so that their mean values are obtained. Using a square of a difference between the respective mean values of the maximum values and the minimum values obtained as a power value, the power value is plotted for each 5 seconds so that a time-series waveform of the power value is produced. In order to read a global change of the power value from this time-series waveform, a slope of the power value regarding a certain time window Tw (180 seconds) is obtained by least-square method. Next, the slope regarding the next time window Tw is similarly calculated in an overlapped time TI (162 seconds) and the calculation results are plotted. A time-series waveform of the slope of the power value is obtained by repeating this calculation (movement calculation) sequentially. On the other hand, the maximum Lyapunov exponent is obtained by applying Chaos analysis to the time-series waveform of the pulse wave, a maximum value is obtained by a smoothing differentiation like the above, and a time-series waveform of a slope of the maximum Lyapunov exponent is obtained by conducting movement calculation.
Then, the time-series waveform of the slope of the power value and the time-series waveform of the slope of the maximum Lyapunov exponent take phases opposite to each other, and a waveform having a large amplitude at a low frequency in the time-series waveform of the slope of the power value is determined as a characteristic signal indicating a sleep prediction and a point at which the amplitude has become small thereafter is determined as a sleep-onset point.
Further, as Patent Literature 4, a system provided with an airbag (air pack) including a three-dimensional solid fabric inserted therein, where the air pack is disposed at a site corresponding to a waist portion of a person, an air pressure fluctuation in the air pack is measured, a biological signal of the person is detected from the time-series waveform of the air pressure fluctuation obtained, and the biological body state of the person is analyzed is disclosed. Further, in Non-Patent Literatures 1 and 2, trials for detecting a biological signal of a person by disposing an air pack sensor along a lumber iliocostal muscle are reported. A pulse wave near a lumber area shows a circulation fluctuation of blood flowing in a descending aorta according to a heartbeat, namely, the motion of the atrium and the fluctuation of the aorta. Incidentally, in the following, a biological signal due to the motion of the atrium and the fluctuation of the aorta, which is obtained from such a dorsal region (lumbar area) is called “aortic pulse wave”. A state change of a person corresponding to a heartbeat fluctuation can be captured in utilization of this aortic pulse wave more accurately than in utilizing the breech pulse wave disclosed in Patent Literatures 1 and 2.